Backlighting a subject against a darker background can create a "rim light" effect that defines their silhouette.
Encouraging them to walk toward or away from the camera helps capture authentic expressions. A popular technique is having them walk away and look back over their shoulder.
To help young teens feel comfortable and look natural, focus on poses that give them something to do with their bodies:
Wear outfits that fit well and make the teen feel confident, as this directly translates to their posture and smile.
Focusing on natural grooming rather than heavy styling often yields the best results for this age group: